In a semiconductor device manufacturing process for forming a laminated integrated circuit structure in a surface of a semiconductor wafer (hereinafter simply referred to as “wafer”) as a substrate, a liquid processing step is provided for carrying out processing of a wafer surface, such as removal of small foreign matter or a natural oxide film from the wafer surface, e.g. by using a cleaning liquid such as a liquid chemical.
As semiconductor devices are becoming more highly integrated, a phenomenon called pattern collapse, which can occur when removing a liquid which has adhered to a wafer surface in such a liquid processing step, is becoming a serious problem. The pattern collapse is a phenomenon in which a liquid, remaining on both sides of a raised portion (i.e. remaining in recessed portions) of a three-dimensional pattern, is unevenly removed upon drying of the wafer surface whereby a surface tension, acting sideways on the raised portion, becomes unbalanced and the raised portion collapses toward the side where a larger amount of the liquid remains.
A method using a fluid in a supercritical state or a subcritical state (hereinafter collectively referred to as a “high-pressure state”) is known as a method to remove a liquid from a wafer surface while preventing the occurrence of pattern collapse. A fluid in a high-pressure state (high-pressure fluid) has a lower viscosity than other liquid and has a high ability to extract a liquid. In addition, no interface exists between a high-pressure fluid and a liquid or a gas in equilibrium with the fluid. Therefore, a liquid adhering to a wafer surface can be removed, without being influenced by the surface tension, by replacing the liquid on the wafer surface with a high-pressure fluid, and then changing the high-pressure fluid into a gas.
The applicant has developed a liquid processing apparatus in which a wafer is subjected to liquid processing, performed by supplying a processing liquid onto the surface of the rotating wafer, in a single-wafer liquid processing unit, and the wafer after the liquid processing, the surface of which is covered with an anti-drying liquid, is transported to a high-pressure fluid processing unit, where the anti-drying liquid is removed from the wafer in a processing container. Patent document 1, for example, teaches the use of HFE (hydrofluoroether), a fluorine-containing organic solvent (described as “fluorine compound” in patent document 1), as an anti-drying liquid and also as a high-pressure fluid from the viewpoints of high replaceability of the high-pressure fluid with a liquid and ability to prevent bringing-in of moisture from liquid processing. A fluorine-containing organic solvent is suitable as an anti-drying liquid also because of its flame retardancy properties.
On the other hand, a fluorine-containing organic solvent such as HFE, HFC (hydrofluorocarbon) or PFC (perfluorocarbon) is more expensive than IPA (isopropyl alcohol) or the like; a loss of such a fluorine-containing organic solvent due to its evaporation during transportation of a wafer leads to an increase in the operating cost. In order to reduce the evaporation loss, fluorine-containing organic solvents having a high boiling point (having a low vapor pressure) are used as an anti-drying liquid and as a high-pressure fluid. A fluorine-containing organic solvent having a high boiling point, however, generally has a high critical temperature, and therefore can decompose in a high-temperature and high-pressure atmosphere to generate hydrogen fluoride (HF), etc., which may cause damage to a wafer. Further, when an anti-drying liquid having a high boiling point is brought into a processing container and the liquid is subjected to processing to remove the liquid, the resulting fluid can condense in the processing container, causing pattern collapse.